The present invention relates to a pipette for replaceable pipette tips.
Pipettes are used in the laboratory in particular for metering liquids. For this purpose, a pipette tip is clamped fast on a seat of the pipette with an upper opening. The seat is mostly a conical or cylindrical projection with respect to a casing of the pipette, onto which a pipette tip can be clamped with the upper opening thereof. The pipette tip can pick up and give out liquid through a lower opening. Air cushion pipettes comprise a displacement equipment for air, which is communicatingly connected to the pipette tip through a hole in the seat. An air cushion is relocated by means of the displacement equipment, so that liquid is sucked into the pipette tip and ejected out from there. For this purpose, the displacement equipment has a displacement chamber with a relocatable limit. The displacement equipment is mostly a cylinder with a piston that can be relocated therein.
After use, the pipette tips are released from the seat and replaced by a fresh pipette tip. Contaminations in subsequent meterings can be avoided through this. Pipette tips have usually an ejection device for ejecting the pipette tips, which permit ejection by actuation of a button without having to touch the pipette tips. Single use pipette tips made of plastics are available at low cost.
The relocatable limit is coupled to a drive equipment, which serves for shifting the piston in the cylinder. The drive equipment has a lifting rod, which can be shifted between an upper and a lower stop with a stop element. In the beginning of the aspiration of air into the displacement chamber, the stop element is situated at the lower stop. In the beginning of the displacement of air out of the cylinder, the stop element rests on the upper stop. The amount of liquid that is picked up or delivered, respectively, depends on the stroke of the relocatable limit, and thus on the stroke of the lifting rod. The stroke volume of the relocatable limit does not correspond exactly to the amount of liquid that is picked up or delivered. As the air column expands somewhat under the weight of the liquid, the stroke volume exceeds the volume of the liquid. The deviation between the stroke volume and the liquid's volume depends in particular on the density and viscosity of the liquid, the temperature, the air pressure and on wetting effects. For instance from the document WO 03/0331515 or U.S. Pat. No. 3,827,305, the entire contents of which is incorporated herein by reference, it is known to calibrate pipettes to a certain metering volume by adjusting the position of an upper stop body.
In fixed volume pipettes, the distance between upper and lower stop is constant. A fixed volume pipette with an upper stop body in the form of a threaded sleeve that is adjustable by a calibration tool is known from the document U.S. Pat. No. 4,020,698, the entire contents of which is incorporated herein by reference,.
In pipettes with adjustable metering volume, the position of the upper stop is variable. Known pipettes have an upper stop body in the form of a threaded spindle, which is adjustable in a spindle nut which is fixedly disposed in the casing. In order to adjust the threaded spindle, there are adjustment equipments, which are coupled to indicating equipments in the form of a counter for indicating the set metering volume. The document DE 43 35 863 C1, the entire contents of which is incorporated herein by reference, describes a pipette wherein the indicating equipments can be uncoupled from the adjustment equipment(s) for calibration.
In the known pipettes, a factory calibration is performed by the pipette manufacturer under standard conditions. In this, double distilled water is pipetted at a temperature of 20 to 25° C. and an air pressure of 1013 mbar. In order to pipette under other conditions, the user must change the factory calibration. Changing the factory calibration and later retrieval of the factory calibration is sumptuous.
The document EP 1 743 701 B1, the entire contents of which is incorporated herein by reference, describes a pipette wherein the lower stop body is held in a holder. An overstroke spring is disposed between the lower stop body and the holder. In addition, there is an adjustment equipment for adjusting the position of the holder with respect to the cylinder. An indicating equipment shows the respective position of the holder. This pipette permits simple calibration by the user, for instance when a liquid is to be metered which has another density or viscosity than double distilled water, or when it is to be worked at different air pressure or temperature. The calibration by the user is made by just only adjusting the lower stop body. The adjustment of the upper stop body and of the indicating equipment made in the factory calibration is not changed at all. The indicating equipment indicates the respective position of the lower stop body. The position of the lower stop body occupied in the factory calibration can easily be found again at any time.
In a practical embodiment, the holder is screwed coaxially to the lifting rod into a carrier which is fixedly disposed in the casing. An indicator ring, disposed coaxially to the lifting rod and having a bevel wheel toothing at the upper edge is rotatably mounted in the carrier. Driving pins project upward from the indicator ring and parallel to the lifting rod, which engage in the axial direction into holes in the bottom of the pot-shaped holder. A pinion is rotatably mounted on the carrier, is engaged with the bevel wheel toothing and has a hexagon socket in a front side. A tool can be inserted in the tool application device through a hole in the casing, in order to rotate the pinion. In this action, the indicator ring is rotated which rotates the holder along via the driving pins. As the holder is screwed into a thread of the carrier, the axial position of the holder in the carrier is changed, and thus also the axial position of the lower stop body. The change of the axial position of the holder with respect to the indicator ring is compensated by the variable introduction depths of the driving pins into the driving holes.
In the known device for user calibration, the precision of the calibration is impaired due to manufacture tolerances and wear of the many component parts. Moreover, the manufacturing expense is high due to the many component parts.
Starting from this, the present invention is based on the task to provide a less sumptuous pipette which offers a user calibration to the user with better calibration precision and being independent of the factory calibration.